Lifters tapping

[farmtruc]

Ok, I just put this truck together. It has Active Fuel Management (AFM) and now I have an issue with it.
The truck starts up fine, sounds good, no tapping, just like a stock truck.
Give it about 20secs or so, and the engine starts making a loud tap.

We pulled the rocker covers and found that the passenger side rockers are all at the same height. Normally, you see some compressed and some up.
The AFM lifters are up, one or two should be down.

How is it that a lifter is fine during start up, then after a little bit, collapses?
It's kind of backwards, normally one is loud and then pumps up and quiets down.

I hate to tear this apart again, any ideas? I just replaced these lifters, could I possibly have a bad set?

btw, it's impossible to put the afm lifters in the wrong location, clock them wrong, or mistake it and put it in a normal lifter location. The plastic retainers that hold the lifters are keyed and have different sized holes for the special lifters. Plus, the entire plastic retainer is keyed, it will only go in one way.

any help is appreciated,
allen

[GasGuzzler]

Allen,

I only talked to you for 30 seconds at the track a while ago but I will say a few things anyway.

There must be problems with L92 (and other engines with AFM) lifters 'cause there are TSB's on them. The document speaks of failed AFM lifters. I've seen none in person.....yet.

I still have a 2007 non-AFM 6.0 with a junk cam (no cams available in the GM parts chain) waiting for parts. During my diagnosis of an engine noise that only happened at 1650 and 2400 RPM (at an audible level) I found that within less than 5 minutes of the engine running, ALL lifters would bleed down enough to close all valves on one head. What's really weird about that? I could pry the valves open with a screw driver in the stock rocker arm and it would not fully close when pressure was released.

Weird indeed.

[farmtruc]

Good info! Sounds like I may have to tear it down again and replace the lifters, again. darn!

It's definitely strange that they bleed down.

[GasGuzzler]

The lifters in the truck I'm wasting away waiting for parts for failed the cam 1st and the lifters (actually just the #3 exhaust lifter) dumped later. The cam looks like it was soft. Let me see if I can find the bulletin on the AFM lifters....

[farmtruc]

I may just replace them with standard lifters, since we're disabling afm.

[ChevyLee]

I have learned to stay away from AFM and Flex Fuel stuff. But im not a professional It gets to complicated to me.

[GasGuzzler]

Subject: GEN III or Gen IV V-8 Engine Misfire

Models: 2008 Buick LaCrosse, Allure
2002-2008 Cadillac Escalade
1999-2008 Chevrolet Avalanche, Express, Silverado, Suburban, Tahoe, Trailblazer
2006-2008 Chevrolet Impala
2006-2007 Chevrolet Monte Carlo
2003-2006 Chevrolet SSR
2005-2008 Chevrolet Corvette
2005-2006 Pontiac GTO
2005-2008 Pontiac Grand Prix
2007 Silverado Classic Body Style
1999-2008 GMC Envoy, Savana, Sierra and Yukon
2007 Sierra Classic Body Style
2003-2008 Hummer H2
2008 Hummer H3

This PI was superseded to include LFA and L92 engines and to change lifter repair strategy. Please discard PIP4138D.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Condition/Concern:

An engine misfire that may be caused by lifter roller or camshaft wear on any 4.8 5.3 6.0 or 6.2 V-8 engine. The misfire described in this PI may be difficult to diagnose, hard to feel, and can occur on any cylinder.
Recommendation/Instructions:

Complete the current diagnostics for any symptom or trouble code found.

If the vehicle is showing misfires on a single cylinder, and the SI diagnostics are inconclusive, there may be a valve train, or camshaft related concern.

A complete compression test, injector balance test, and a cylinder leakage test must be completed and documented. If these diagnostics are inconclusive, continue with the information found in this PI.

Valve train concerns that are causing difficult to diagnose conditions are listed below. The tests and/or inspections should be completed by the easiest to find to the most difficult.

1. Valve carbon build up from poor fuels causing valve sticking/valve seating concerns.
2. Broken Valve Springs
3. Valve sticking due to tight valve to guide clearances.
4. Valve seat leaking due to valve face to seat contact.
5. Lifter lock pin not locking (only on the Active Fuel Management (AFM) equipped engines).
6. Lifter roller pitting or flat spotting.
7. Lifter roller damage and/or cam lobe wear.

Thoughts on root causing some of the misfire concerns that may be encountered.

1. The use of Top Tier fuels, and GM fuel system treatment plus, may alleviate many of the valve carbon deposit concerns. The use of Upper Engine and Fuel Injector Cleaner diluted with gasoline may be introduced into the throttle body as an induction cleaner. Mixing instructions from bulletin number 03-06-04-030 can be used. Extreme care must be taken not to hydrolock the engine.
2. Valve to guide clearance concerns may be temperature related. To diagnose this concern the technician can use SI procedures for On Vehicle Valve Stem Oil Seal and Valve Spring Replacement. When the spring has been removed from the valve, wrap a rubber band tightly around the valve stem near the tip of the valve. (This step will ensure valve cannot be dropped into the combustion chamber). Release the air pressure in the cylinder that is holding the valve closed. Work the valve up and down in the guide while turning the valve a complete 360 degrees. Any binding felt in the valve would suggest a tight or damaged valve guide. Repair valve guide concerns as needed.
3. Valve seating/sealing concerns should be found with cylinder leakage testing. If a valve face or valve seat concern has been found, repair or replace only the effected cylinder head and valves.
4. If a lifter lock pin concern has been found and the rocker arm is not being actuated correctly, replace only the effected lifter. Inspect the old lifter roller for damage. Camshaft lobe inspection should also be completed when the lifter has been removed.
5. If a lifter has large flat spots or pitting, the camshaft and all lifters will need to be replaced.

If a camshaft or lifter concern has been verified, complete the current SI procedures to repair the vehicle. Engineering has collected samples of returned parts and is investigating camshaft/lifter concerns. This PI will be updated when new information becomes available.

Note: If working with an AFM equipped engine, inspect the VLOM screen for debris. The oil screen is located in the VLOM oil inlet passage, below the oil pressure sensor. Clean the VLOM screen as necessary during the repair.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

[GasGuzzler]

There are some details there that are easy to overlook.

Also, I've seen/heard of massive gains by camming these well and ignoring AFM. I heard of L92's with bigger cams and LT's with something relative to a Nelson tune making big numbers with a non-AFM cam. Not sure what it takes but it seems like a non-AFM cam, non-AFM lifters in the stock lifter bores, and some time on the pump.

Sounds like something you can do from what I hear.

Have fun!

[GasGuzzler]

Now that I think about it, doesn't seem possible to use non-AFM lifters in the AFM block. Therefore the cam change idea doesn't help because it's the lifters you're fighting.

Is the entire valvetrain stock? Stock non-adjustable rockers, pushrods, valves, cam, etc.? Can you feel or check for lash while it's running with the rocker covers off? That's how I found the bad lobe on the engine described earlier.

You might try some slightly longer pushrods to see if you can get the plunger down in the lifter farther and eliminate the lash BUT THIS COULD CAUSE MASSIVE PROBLEMS WITH THE AFM LIFTERS (only a guess).

[Madd Maxx]

How is it that a lifter is fine during start up, then after a little bit, collapses?
It's kind of backwards, normally one is loud and then pumps up and quiets down.

As I understand this system, there are two solenoids in the manifold for each deactivated cylinder. There are also two separate oil passages to each lifter. One solenoid controls the the lifter pin movement, and the other provides oil for lash adjustment. One passage provides oil to move the pin and the other provides oil to take up the lash when the inner lifter cylinder stops moving.

It sounds like you might have a problem with the pin activation solenoids. They may be energizing at the wrong time and deactivating the cylinders. If the lash adjustment solenoid is not activating at the same time, you would have the condition you describe.

Just a thought.